In the prior art, there has been produced a heater having on a substrate composed mainly of Al.sub.2 O.sub.3 an electron-conductive pattern desired to generate heat. However, when current (direct current) is continued to be applied through the heater, blackening or peeling-off occurs in the vicinity of the cathode terminal. There occur an increase in the resistance and hence partial heat generation, which may result in deterioration of the durability of the heater.